[SI-LIST] Re: Number of points for your VNA
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 13 Jan 2014 06:39:15 -0800
Scott said 2 round trips = 4 L. 2L is just one round trip. It is not a
question of phase as old energy rattling around due to imperfect end
matches.
Steve.
On 1/13/2014 6:32 AM, FU, Yejun wrote:
> Hello Scott:
> I think it only takes about 2 rounds trip to dissipate not 4
> rounds, which is more easy to understand as from point view of phrase
> aliasing. To avoid it, you have to make sure your phase rotating is less than
> 180 degrees between your two adjacent frequency points.
>
> Phase Theta¾ta*L here, beta=2Pi/lambda,
> lambda=c/[f*sqrt(dielectric constant)],
>
> phase variation, delta Theta, must less than Pi (180).
>
> Then, combine all above relations then you will have:
>
> Delta f < 1/[2*L* propagation delay]
>
> So it is 2L
>
>
> Johnny
>
> From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
> Sent: Tuesday, January 07, 2014 10:10 AM
> To: FU, Yejun
> Cc: lingyunwang@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Number of points for your VNA
>
> The rule of thumb to absolutely avoid aliasing in the time domain is to
> assume that residual energy in the interconnect takes 2 round trips to
> dissipate. for 30" of trace in a low loss dielectric propagation is
> approximately 160 ps per inch. Two round trips would therefore be 30 X 4 X
> 160 ps = 19.2 ns of delay. That corresponds to a frequency of 1/19.2ns = 52
> MHz.
>
> The maximum frequency domain step size should be no more than 50 MHz (a nice
> round number). Since you have enough resolution on the VNA to go down to 10
> MHz, and I happen to like step sizes that give sample points that are
> multiples of 5 and/or 10, you can set your VNA to sweep from 10 MHz to 40 GHz
> in 10 MHz steps, which would be 4000 points.
>
> A reasonable setting for your VNA is to sweep from 10 MHz to 40 GHz with 4000
> points. This will give you all the resolution that you need for any
> interconnect analysis in your system.
>
> On Tue, Jan 7, 2014 at 8:47 AM, FU, Yejun
> <yfu@xxxxxxxxx<mailto:yfu@xxxxxxxxx>> wrote:
> Hello Lyn:
>
> It's a 30inchs high speed differential trace, I set the span
> from 10MHZ to 40GHz, with number of point 160001(please don't ask me why). I
> have long argument and talking with many people then I post my question to
> you guys.
>
> Johnny
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
> [mailto:si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>] On
> Behalf Of Lingyun Wang
> Sent: Monday, January 06, 2014 12:20 PM
> To: fuyejun@xxxxxxxxx<mailto:fuyejun@xxxxxxxxx>
> Cc: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Number of points for your VNA
>
> Hi Johnny,
> Unquestionably, the larger the number of points is, the better the accuracy
> will be. The penalty along with that is the unendurable calibration time,
> restless waiting for each measurement being done, and the large data file if
> you want to keep it a record.
>
> What kind of measurement that you are working on? If it is for PDN, the log
> scale will be recommended since the low frequency points really matters. If
> it is for high speed with long length DUT, the linear scale at high frequency
> range to guarantee no missing points.
>
> There is a list of questions needed to be known before finding the best
> sampling points: such as the length of DUT, frequency range to be
> investigated, purpose of the measurement...
> It is the trade off between the measurement efficiency (mostly the time
> consumed in the each measurement) and the accuracy. If the DUT is less than
> 50inch, the log scale with 501pt/dec would be a safe zone for kind of
> measurement.
>
> Best,
> Lyn
>
>
>
>
> On Mon, Jan 6, 2014 at 7:17 AM, Yejun Fu
> <fuyejun@xxxxxxxxx<mailto:fuyejun@xxxxxxxxx>> wrote:
>
>> Hi all:
>>
>> I want to know how to define your VNA's # of points, some
>> person told me it is depends on the your DUT's length, some times if
>> you have fibers in your DUT you have to change your points value.
>> Could any one help to clear this question?
>>
>> Thanks
>>
>> Johnny
>>
>>
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> --
>
> Scott McMorrow
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